Sex differences in the acoustic structure of an alarm vocalization in a monomorphic primate: wild silky sifakas (Propithecuscandidus) of northeastern Madagascar

The distribution and functions of sex differences in adult-primate vocal acoustics are not well known, but are often considered to reflect factors like body-size dimorphism and mating strategies. While applicable to species such as chimpanzees, bonobos, baboons, vervets, and rhesus monkeys, these explanations cannot readily account for also finding sex differences in vocalizations of monomorphic callitrichidae or in the long calls of monogamous sportive lemurs. We tested for sex differences in the “Zzuss” vocalizations of another monomorphic lemur, the silky sifaka. This diurnal species lives in multi-male groups in mid-altitude, primary montane rainforest in north-eastern Madagascar. All-occurrence audio-recording of three groups over the course of 14 months yielded 690 vocalizations by 9 adults (4 females, 5 males). Temporal, amplitude, and frequency measurements were extracted at both call and intra-call levels using spectrographic, spectral, and waveform analyses. Mann-Whitney U tests (P < 0.01) revealed significant sex differences in a number of measures, which in turn allowed successful prediction of vocalizer-sex in 65% of cases using binary logistic regression (P < 0.001). Females emitted calls with higher measured amplitudes and fundamental frequencies, which might simply reflect higher arousal. However, both the pitch difference and a lower first formant in females implicate vocal-fold and vocal-tract size dimorphism, respectively. Results add to previous evidence suggesting that dimorphism in vocal production is not an obligate reflection of sex differences in general anatomy or behavior.